This invention relates to apparatus for removing fine solid or liquid particles from a gas stream.
There are many situations where dust must be removed from a gas stream, such as in removing dust from the exhaust of a power plant prior to the exhaust reaching the atmosphere, and in removing dust from air utilized to ventilate a mine shaft. Where only a moderate proportion of the dust particles, such as about 90%, must be removed, and where it is not necessary to remove very fine particles, such as those below 10 microns in diameter, a cyclone type apparatus may be utilized. Such a cyclone device can include a chamber where the particle-laden gas is introduced in circulatory flow thereabout, and a cone-shaped constrictor causes the gas to circulate more rapidly so as to centrifugally force particles to the outside of the stream. The constricted area can be followed by a diffusing chamber which permits the particle-laden air near the outside of the constricted area, to undergo settling so the particles settle out, while air at the center and middle of the constricted area, which is largely free of particles, can be passed into the atmosphere. Passage into the atmosphere with a reverse flow can be accomplished with a vortex finder tube that passes back through the center of the vortex. Such cyclone separators cannot remove more than about 95% of the particles, regardless of how fast the air circulates in the constricted region thereof, because some dust which settles out is always reintrained in the airstream by reason of turbulence. Further, they have not proven effective in removing particles of less than about 10 microns diameter. The cyclone separators usually are operated to generate a G force of no more than about 1000 G's, because higher speeds create increased turbulence which throws settling particles back into the middle of the air stream.
In application where a very high proportion of the dust, such as over 98% thereof, must be removed, and where even fine particles of less than 10 microns in diameter must be removed, venturi scrubbers have been utilized. Such scrubbers direct air in axial flow through a venturi, wherein a water mist is applied to cause the water mist to entrain particles. The venturi passage is followed by a demisting device, such as a cyclone or mesh pad, to remove the particle-water slurry from the airstream. Such devices have a disadvantage in that they require a considerable flow-through of water, such as 3 to 5 gallons of water flow per 1,000 cubic feet of air flow and result in a high rate of evaporation. This has the disadvantage that such devices may not be useful where water sources are limited. Also, the devices necessarily generate considerble outflow of a particle-water slurry, so that storage areas or settling tanks must be provided to handle such an outflow. In fact, the cost of equipment for handling the water-particle outflow of the scrubber can easily constitute the principal portion of the cost of a scrubber installation.